1. Field of the Invention
The present invention relates to a circuit that detects leakage mainly of a charging/discharging circuit of a rechargeable battery, and particularly to a leakage detection circuit for an electric vehicle that detects leakage of a power supply device that powers a motor for running the electric vehicle, such as hybrid car and an electric car.
2. Description of the Related Art
High voltage is required to increase the output of a power supply device for driving an electric vehicle. The reason is that output is proportional to the product of voltage and current. For example, the output voltage of a power supply device for driving a hybrid car or an electric car is very high voltage of 200 V or more. Since, in a high-voltage power supply device, leakage causes a serious harmful effect, the device is not connected to the ground in consideration of safety. In order to prevent leakage, it is necessary for the power supply device that is not connected to the ground to detect a leakage resistance value. The leakage resistance value is a resistance value between the power supply device and the ground. FIG. 1 corresponds to FIG. 1 in Japanese Patent Laid-Open Publication No. 2003-169401, and shows a detection circuit for detecting the leakage resistance value of a power supply device. A leakage detection circuit 50 shown in this Figure has a leakage detection resistor 51, a leakage detection switch 52, and a voltage detection circuit 53 that detects the voltage produced in the leakage detection resistor 51. When a leakage resistance Rr exists, in the state where the leakage detection switch 52 is turned ON, a current flows through the leakage detection resistor 51. Accordingly, detection of the voltage of the leakage detection resistor 51 can detect leakage.
However, it is difficult for the leakage detection circuit shown in FIG. 1 to accurately calculate the leakage resistance value. The inventor has developed a leakage detection circuit shown in FIG. 2 to solve this disadvantage (see Japanese Patent Laid-Open Publication No. 2005-338010).
The leakage detection circuit shown in FIG. 2 includes a battery pack 70, leakage detection resistors Ra and Rb, a voltage detector 72, and voltage detection circuits 73. The battery pack 70 includes a plurality of batteries 71 that are connected to each other in series. Each of the leakage detection resistors Ra and Rb is connected in series between the ground, and each of any two high and low voltage side terminal points in the batteries 71. The voltage detector 72 detects the voltage values of the battery terminals on the high and low voltage sides as Vg11(t) and Vg12(t), respectively, at timing t. Each of leakage detection switches SW1 and SW2 is connected in series between the leakage detection resistors Ra and Rb. One of the voltage detection circuits 73 detects a voltage value VI11(t) that is produced in the leakage detection resistor Ra that is connected to the leakage detection switch SW1, when one leakage detection switch SW1 that is connected to the battery terminal on the high voltage side is closed and the other leakage detection switch SW2 is opened at timing t. The other voltage detection circuit 73 detects a voltage value VI12(t) that is produced in the leakage detection resistor Ra that is connected to the leakage detection switch SW2, when the other leakage detection switch SW2 that is connected to the battery terminal on the low voltage side is closed and the one leakage detection switch SW1 is opened at timing t. In addition to this, the leakage detection circuit further includes a leakage calculator 74 that calculates the combined value RI of the leakage resistors based on following Equation 1.
                              R          I                =                                            R              a                                                                                            V                                          I                      ⁢                                                                                          ⁢                      12                                                        ⁡                                      (                                          t                      2                                        )                                                                                                              V                                              g                        ⁢                                                                                                  ⁢                        11                                                              ⁡                                          (                                              t                        2                                            )                                                        -                                                            V                                              g                        ⁢                                                                                                  ⁢                        12                                                              ⁡                                          (                                              t                        2                                            )                                                                                  -                                                                    V                                          I                      ⁢                                                                                          ⁢                      11                                                        ⁡                                      (                                          t                      1                                        )                                                                                                              V                                              g                        ⁢                                                                                                  ⁢                        11                                                              ⁡                                          (                                              t                        1                                            )                                                        -                                                            V                                              g                        ⁢                                                                                                  ⁢                        12                                                              ⁡                                          (                                              t                        1                                            )                                                                                                    -                      (                                          R                a                            +                              R                b                                      )                                              Equation        ⁢                                  ⁢        1            
where t1 and t2 are different time values t.
According to this construction, it is possible to easily obtain the leakage resistance value RI, and to calculate the leakage resistance value RI of the whole circuit even if leakage occurs at two or more points.